This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-45174 filed on Feb. 21, 2002.
The present invention relates to a capacitive acceleration sensor that measures an acceleration using two capacitances that exclusively increase or decrease in response to the acceleration.
FIG. 6 is a plan view of a proposed capacitive acceleration sensor of this type. The sensor in FIG. 6 includes a supporting substrate 11, which is not illustrated in FIG. 6, fixed members 30, 40, and a movable member 20. The fixed members 30, 40 are located above a surface of the supporting substrate 11. The fixed members 30, 40 are supported by the supporting substrate 11 to be stationary with respect to the supporting substrate 11. The movable member 20 is also located above the surface. The movable member 20 is supported by the supporting substrate 11 to be movable with respect to the supporting substrate 11. The fixed and movable members 20, 30, 40 are formed from a semiconductor layer, which has been formed on the supporting substrate 11, by etching the layer to form a trench.
The movable member 20 includes a weight 21, which is mechanically and electrically linked to spring members 22. The movable member 20 can move along directions Y in FIG. 6, which is parallel to the surface of the supporting substrate 11, in response to the acceleration of the sensor with the spring-like action of the spring members 22. The movable member 20 also includes comb-shaped electrodes 24. The comb-shaped electrodes 24 are respectively located on left and right ends of the weight 21 in FIG. 6. Each comb-shaped electrode 24 includes four movable electrode beams, which are substantially-straight. As shown in FIG. 6, the comb-shaped electrodes 24 are substantially symmetrical with respect to the weight 21.
On the other hand, each fixed member 30, 40 includes a comb-shaped fixed electrode 32, 42, which is interleaved with each corresponding movable electrode 24, as shown in FIG. 6. Each fixed electrode 32, 42 includes four fixed electrode beams, which are substantially-straight. As shown in FIG. 6, there is a left clearance d1xe2x80x2 between the left movable electrode 24 and the left fixed electrode 32, more specifically between each left movable electrode beam and the corresponding left fixed electrode beam, and a left capacitance CS1 is formed between the left movable electrode 24 and the left fixed electrode 32. On the other hand, there is a right clearance d2xe2x80x2 between the right movable electrode 24 and the right fixed electrode 42, more specifically between each right movable electrode beam 24 and the corresponding right fixed electrode beam 42, and a right capacitance CS2 is formed between the right movable electrode 24 and the right fixed electrode 42.
The movable electrodes 24 move along the directions Y with the weight 21 in response to a force that acts on the movable member 20 along the directions Y. In the proposed sensor, when the force is zero, the left and right clearance d1xe2x80x2, d2xe2x80x2 are substantially equal to each other and so are the left and right capacitances CS1, CS2. In addition, when the movable electrodes 24 move, the capacitances CS1, CS2 change in a manner that one of the capacitances CS1, CS2 increases while the other decreases. Moreover, the force can be correlated to an acceleration of the sensor. Therefore, the acceleration can be measured based on the difference (CS1xe2x88x92CS2 ) between the capacitances CS1, CS2.
The proposed capacitive acceleration sensor is used, for example, in an automobile, in which the sensor is mounted such that the surface of the supporting substrate 11 is approximately horizontal to the ground. Therefore, the force that acts on the movable member 20 along the directions Y is in direct proportion to the acceleration of the automobile. There is a need today, however, to use a capacitive acceleration sensor for measuring an acceleration in the vertical directions to the ground. When the proposed capacitive acceleration sensor is used for such an application, the proposed sensor needs to be attached on an object, the acceleration of which is being measured, such that the surface of the supporting substrate 11 is approximately vertical to the ground.
In that case, when an acceleration is measured using the sensor, the force includes a substantially constant force, which is caused by the gravity, in addition to a variable force, which is proportional to the acceleration. Therefore, there is a constant positional shift in the movable electrodes 24 due to the gravity along the directions Y. For example, when the sensor in FIG. 6 is mounted such that the directions Y become vertical to the ground while the lower side of the supporting substrate 11 in FIG. 6, on which electrode pads 25a, 31a, 41a are located, become more distant from the ground than the upper side thereof, the left clearance d1xe2x80x2 increases, and the left capacitance CS1 decreases. At the same time, the right clearance d2xe2x80x2 decreases, and the capacitance CS2 increases.
As a result, when an acceleration vertical to the ground is measured, the capacitance difference corresponding to the gravity is included in the output from the sensor in addition to the capacitance difference corresponding to the acceleration. If the acceleration is comparable in magnitude to the gravity, the margin of error in measurement results is not negligible. Moreover, the output can be susceptible to errors or would be saturated if the capacitance difference corresponding to the gravity is out of the detection range of the proposed sensor.
The proposed sensor would be susceptible to the same problem as long as the proposed sensor is under a constant force such as the gravity along the directions Y, or the movement directions of the movable member 20, when in use.
The present invention has been made in view of the above aspects with an object to address the above-described issue with the proposed capacitive acceleration sensor.
A capacitive acceleration sensor according to the present invention includes a supporting substrate, a movable member, and two fixed members. The movable member is located above a surface of the supporting substrate and supported by the supporting substrate to move with respect to the supporting substrate along predetermined directions parallel to the surface in response to a force that acts on the movable member along the predetermined directions. Each fixed member is located above the surface and supported by the supporting substrate to be stationary with respect to the supporting substrate under the force.
Two capacitances are formed between the movable member and the fixed members. One of the capacitances increases while the other decreases when the movable member moves in response to the force. The force includes a substantially constant force and a variable force when an acceleration is measured using the sensor. The variable force is proportional to the acceleration. The acceleration is measured on the basis of a difference in quantity between the capacitances. The capacitances are different in quantity from each other when the force that acts on the movable member is zero to reduce a difference in quantity between the capacitances that corresponds to the substantially constant force.
As a result, it is possible to reduce substantially down to zero the difference in quantity between the capacitances that corresponds to the substantially constant force. Therefore, the capacitive acceleration sensor according to the present invention is capable of measuring appropriately an acceleration even if the movable member is under a constant force along the predetermined directions, or the movement directions of the movable member, when in use.